The present invention relates to a printer or similar electrophotographic image forming apparatus for forming images on optical disks or similar synthetic resin sheets. More particularly, the present invention relates to an image forming apparatus for forming a toner image on a synthetic resin sheet while conveying the sheet with a conveyor, and causing a fixing device to fix the toner image on the sheet with heat.
Today, an electrophotographic image forming apparatus capable of forming attractive full-color images on, e.g., paper sheets and OHP (Over Head Projector) sheets are extensively used. Further, there has been proposed in various forms an image forming apparatus of the type forming an image on one surface of an optical disk, e.g., a CD, (Compact Disk), a CD-RW (CD ReWritable), a LD (Laser Disk) or a DVD (Digital Versatile Disk) or similar synthetic resin sheet, e.g., on the protection layer surface of a CD. It has been customary with this type of image forming apparatus to use offset printing or screen printing. However, the problem with offset printing or screen printing is that a master corresponding to a desired image must be produced by an extra process beforehand. As a result, the apparatus lacks efficiency when producing many kinds of images or increases cost when producing a small number of images.
In light of the above, Japanese Patent Laid-Open Publication Nos. 5-212857 and 11-167312, for example, each propose an electrophotographic label printer for optical disks operable in the same manner as the traditional image forming apparatus for paper sheets or similar recording media. The label printer does not need masters and therefore the extra process for producing them. The electrophotographic image forming process includes an image forming step for transferring a toner image from an image carrier to a synthetic resin sheet, and a fixing step for fixing the toner image on the sheet. For the fixing step, use is made of a heat roller that heats the toner image carried on the synthetic resin sheet while pressing it against the sheet.
More specifically, the synthetic resin sheet carrying the toner image thereon is conveyed to a fixing position where the sheet contacts the heat roller over a preselected nip. At the fixing position, the heat roller heats the toner on the synthetic resin sheet while pressing it against the sheet. As a result, the toner image remains fixed on the synthetic resin sheet even after the sheet has moved away from the fixing position.
However, a series of experiments showed that the toner image formed on the synthetic resin sheet by the conventional apparatus suffered from irregularity, peeling, short gloss and other various defective fixation. We experimentally found that the defective fixation was ascribable to the following causes.
First, heat expected to cause the toner to melt during fixation is presumably short. A full-color image forming apparatus, for example, includes a fixing device made up of a heat roller and a backup roller pressed against the heat roller. The heat roller and backup roller each have a heater thereinside. These two rollers heat opposite sides of a paper sheet at the same time while conveying the paper sheet and can therefore sufficiently heat toner deposited on the paper sheet.
Assume that the above-described fixing device is used to fix a toner image formed on, e.g., the protection layer surface of an optical disk. Then, the backup roller, conveying the disk in cooperation with the heat roller, contacts the surface of the disk opposite to the projection layer surface (recording surface hereinafter). It is therefore likely that dust and other impurities deposited on the backup roller are transferred to the recording surface. Further, silicone oil or similar parting agent coated on the heat roller is transferred to the backup roller and therefore to the recording surface of the disk.
The impurities deposited on the recording surface of the disk, as stated above, obstruct the accurate read-out of data when the disk is played. It is therefore impractical to cause the backup roller to contact the recording surface of the disk or heat it. That is, the heat roller heats the protection layer surface of the disk alone. This is why the toner on the protection layer surface of the disk cannot be sufficiently heated, compared to toner on a paper sheet that can be heated from opposite sides, resulting in defective fixation.
Second, when a paper sheet is used as a recording medium, the toner melted by the heat roller can be pressed into gaps between the filaments of paper and therefore peels off little. However, it is difficult to fix toner on the surface of a disk or similar synthetic resin sheet that is smoother than the surface of a paper sheet. While the toner should therefore be sufficiently heated and firmly fixed on the synthetic resin sheet, heat for causing the toner to melt is locally short. This kind of defective fixation is likely to occur even with OHP sheets or simple plastic sheets.
Third, an optical disk or similar synthetic resin sheet has greater thermal capacity than, e.g., a paper sheet and cannot be heated as easily as a paper sheet. Specifically, the heat roller heats the disk either directly or via toner deposited on the disk. Therefore, a temperature difference between the disk and the toner during fixation is greater than a temperature difference between a paper sheet and toner. Consequently, heat fed from the heat roller to the toner is easily transferred to the disk, i.e., it cannot be efficiently fed to the toner. Moreover, the disk includes a metallic reflection layer having high thermal conductivity and adjoining the protection layer surface. The reflection layer extends over the entire disk and therefore has higher thermal conductivity than, e.g., a paper sheet, causing the heat fed from the heat roller to scatter. In this manner, for a given amount of heat, more heat is lost from the disk than from a paper sheet at a nip for fixation.
Even a synthetic resin sheet not including a reflection layer or similar layer having high thermal conductivity has greater thermal capacity than a paper sheet. This also results in the above-described defect.
To obviate defective fixation described above, higher fixing temperature may be assigned to a synthetic resin sheet than to, e.g., a paper sheet. This, however, aggravates power consumption. Alternatively, a longer fixing time for a unit area may be assigned to a synthetic resin sheet than to a paper sheet. This kind of scheme insures sufficient heat and thereby obviates the above occurrence. However, if the overall process speed for image formation is lowered in order to implement a long fixing time, then a period of time necessary for an image forming cycle increases, reducing the number of prints for a unit period of time.
Assume that the heat roller has a circumferential length greater than the length of a synthetic resin sheet in the direction of conveyance and therefore makes more than one rotation before the sheet arrived at the nip leaves the nip. This brings about another problem that a portion of the heat roller lost heat at the nip, i.e., lowered in temperature again contacts the surface of the synthetic resin sheet. Such a portion of the heat roller cannot sufficiently heat the synthetic resin sheet and therefore toner deposited thereon. This also results in defective fixation described above.
Moreover, defective fixation is apt to occur when the heat roller has an axial dimension in a direction perpendicular to the direction of conveyance that is smaller than the dimension of a synthetic resin sheet in the same direction. Such defective fixation may be obviated if the heat roller is provided with as large an area as possible in both of the circumferential and axial directions. This, however, requires the heat roller to be wastefully heated and is therefore undesirable from the energy consumption and space requirement standpoint.
On the other hand, in the image forming apparatus of the type causing the heat roller to contact a synthetic resin sheet, which is being conveyed by the conveyor, slip between the heat roller and the sheet disturbs the toner image carried on the sheet and thereby lowers image quality. To solve this problem, the peripheral speed of the heat roller and the conveying speed of the conveyor must be accurately matched to each other. However, when drive sources assigned to the heat roller and conveyor, respectively, are different in construction as conventional, it is difficult to accurately match the above two speeds. This is also true when the dimensional accuracy of a drive mechanism assigned to the heat roller is irregular.
While the above description has concentrated on a fixing member implemented as a heat roller, defective fixation is apt to occur even when the fixing member is implemented as, e.g., an endless belt.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 11-305560.
It is a first object of the present invention to provide an electrophotographic image forming apparatus capable of fixing a toner image carried on a synthetic resin sheet or an optically writable, data recording medium over a period of time long enough to obviate defective fixation while producing as great a number of prints as possible.
It is a second object of the present invention to provide an electrophotographic image forming apparatus capable of fixing a toner image carried on a synthetic resin sheet or an optically writable, data recording medium while obviating wasteful energy consumption and saving space.
It is a third object of the present invention to provide an electrophotographic image forming apparatus capable of fixing a toner image carried on a synthetic resin sheet or an optically writable, data recording medium while accurately matching the peripheral speed of a heat roller and the conveying speed of a conveyor.
An image forming apparatus of the present invention includes an image carrier for forming a toner image thereon. A conveyor conveys a synthetic resin sheet. An image transferring device transfers the toner image from the image carrier to the synthetic resin sheet being conveyed by the conveyor. A fixing device is located downstream of the image carrier in a direction of sheet conveyance for fixing the toner image transferred to the synthetic resin sheet. The fixing device includes a heat roller capable of contacting the synthetic resin sheet. The heat roller is freely rotatable and caused to rotate by the synthetic resin sheet being conveyed by the conveyor when the sheet contacts the heat roller.